IB/core: Expose and rename ib_find_cached_gid_by_port cache API

Sometime consumers might want to search for a GID in a specific port.
For example, when a WC arrives and we want to search the GID
that matches that port - it's better to search only the relevant
port.
Exposing and renaming ib_cache_gid_find_by_port in order to match
the naming convention of the module.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>